1. Field of the Invention
This invention relates generally to transmitting rotating power in a motor vehicle driveline. More particularly, it pertains to controlling a torque distribution clutch while braking a vehicle on a slippery road or any surface having a relatively low coefficient of friction.
2. Description of the Prior Art
A four wheel drive (4WD) powertrain of a motor vehicle includes an engine, a transmission for producing multiple forward speeds and reverse drive, and a transfer case, which continually connects the transmission output to a rear drive shaft and selectively connects the transmission output to both the front wheels and rear wheels when a four-wheel drive mode of operation is selected. A transfer clutch located in the transfer case directs more torque to the front wheels as the torque capacity of the clutch is increased in response to actuation by a clutch servo, which is usually controlled by an electronic controller.
When the wheel brakes of a vehicle equipped with a 4WD powertrain are applied while the vehicle is on a slipper road surface, transient vibrations can be produced in the powertrain due to these conditions. Wheel brake biasing with electronic brake distribution along with an increased anti-lock brake system speed threshold can cause the average front wheel speeds to decelerate much faster than the rear average wheel speeds. While this speed differential is present, the wheel slip control of the 4×4 control system can cause an increase in the transfer clutch torque capacity, thereby causing the rear wheels to lock up cyclically, repeated bumps, and torque disturbance.
There is a need in the industry to avoid such objectionable operation by controlling the transfer clutch when the vehicle operates under conditions that cause the objectionable transients.